Cutting down the number of calories you
consume in a day is the cornerstone of losing excess weight. As
a rough rule of thumb, here is how calories translate to weight:

One pound of fat equals about 3,500
calories. Therefore, one could
lose a pound a week by cutting about 500 calories a day.
Naturally, the more calories you cut, the faster the weight
loss. However, very-low calorie diets have been associated
with some serious health consequences. (See below.)

To determine your own personal daily
calorie requirement, multiply the
number of pounds of your target (ideal) weight by 12 to 15
calories. This gives a range that lets you adjust for
gender, age, and activity levels. For instance, a 50-year
old woman who wants to maintain a weight of 135 pounds and
be mildly active might require only 12 calories per pound
(1,620 calories a day). A 25-year old female athlete who
wants to maintain the same weight might require 25 calories
per pound (2,025 calories a day).

Warning on extreme diets

Extreme diets of less than 1,100 calories per
day carry health risks and are often followed by bingeing or
overeating and a return to an obese state. Such diets often have
insufficient vitamins and minerals, which must then be taken as
supplements. Most of the initial weight loss is in fluids.
Later, fat is lost, but so is muscle, which can account for more
than 30% of the weight loss.

No one should be on severe diets longer than
16 weeks or fast for more than two or three days. Severe dieting
has unpleasant side effects, including fatigue, intolerance to
cold, hair loss, gallstone formation, and menstrual
irregularities. There have been rare reports of death from heart
arrhythmias when liquid formulas did not have sufficient
nutrients. Those whose diets include a high intake of fluids and
much reduced protein and sodium are at risk for hyponatremia
(low sodium), which can cause fatigue, confusion, dizziness, and
in extreme cases, coma.